Buried source-drain contact for integrated circuit transistor devices and method of making same

ABSTRACT

An integrated circuit transistor is formed on a substrate. A trench in the substrate is at least partially filled with a metal material to form a source (or drain) contact buried in the substrate. The substrate further includes a source (or drain) region in the substrate which is in electrical connection with the source (or drain) contact. The substrate further includes a channel region adjacent to the source (or drain) region. A gate dielectric is provided on top of the channel region and a gate electrode is provided on top of the gate dielectric. The substrate may be of the silicon on insulator (SOI) or bulk type. The buried source (or drain) contact makes electrical connection to a side of the source (or drain) region using a junction provided at a same level of the substrate as the source (or drain) and channel regions.

FIELD OF THE INVENTION

The present disclosure generally relates to integrated circuits and, in particular, to integrated circuits with transistors formed with a small pitch.

BACKGROUND

Those skilled in the art understand that integrated circuit dimensions are becoming increasingly smaller. Indeed, there is a desire to produce integrated circuit devices, such as transistors, which exhibit a pitch scaling <100 nm and still further <50 nm. As the pitch scaling continues to decrease, it becomes increasingly more difficult to make electrical contact to transistor source, drain and gate regions from above the transistor (as is commonly done in the prior art). The dimensions of the gate contact, made from above the transistor, may exceed the length dimension of the transistor and extend above the source region and drain region. Such extension can effectively block access to the source and drain regions from above the transistor.

There is accordingly a need in the art for an alternatively means to make electrical contact to transistor source and drain regions in instances of increasingly smaller transistor pitch scaling.

SUMMARY

In an embodiment, an integrated circuit transistor comprises: a substrate including a trench; a metal material at least partially filling the trench to form a source contact buried in the substrate; a source region in the substrate in electrical connection with the source contact; a channel region in the substrate adjacent the source region; a gate dielectric on top of the channel region; and a gate electrode on top of the gate dielectric.

In an embodiment, a method comprises: forming a trench in a substrate; at least partially filing said trench with a metal material form a source contact buried in the substrate but exposing a side wall of the substrate; doping the exposed side wall of the substrate to define a source region adjacent a channel region; providing a gate dielectric on top of the channel region; and forming a gate electrode on the gate dielectric.

The foregoing and other features and advantages of the present disclosure will become further apparent from the following detailed description of the embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the disclosure, rather than limiting the scope of the invention as defined by the appended claims and equivalents thereof.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments are illustrated by way of example in the accompanying figures not necessarily drawn to scale, in which like numbers indicate similar parts, and in which:

FIG. 1 is a cross-sectional diagram of a plurality of transistors which utilize a buried contact made to the transistor source-drain regions;

FIGS. 2A-2M illustrate process steps for forming the integrated circuit shown in FIG. 1;

FIG. 3A is a plan view of the integrated circuit shown in FIG. 1;

FIG. 3B is a cross-sectional view of the integrated circuit shown in FIG. 1;

FIG. 4 is a cross-sectional diagram of an alternative embodiment of a plurality of transistors which utilize a buried contact made to the transistor source-drain regions;

FIGS. 5A-5G, along with FIGS. 2E-2M, illustrate process steps for forming the integrated circuit shown in FIG. 4;

FIG. 6 is a cross-sectional diagram of an alternative embodiment of a plurality of transistors which utilize a buried contact made to the transistor source-drain regions; and

FIGS. 7A-7I, along with FIGS. 2E-2M, illustrate process steps for forming the integrated circuit shown in FIG. 6.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is now made to FIG. 1 which illustrates a cross-sectional diagram of a plurality of transistors utilizing a buried contact made to the transistor source-drain regions. Each transistor 10 includes a gate region 12, a source region 14 and a drain region 16. A gate contact 18 extends from above the transistor to make electrical contact to the gate region 12. A source-drain contact 20 extends from below the transistor to make electrical contact to the source region 14 and/or drain region 16 through a junction 22. The transistors 10 are formed on and in a substrate 24. In the illustrated embodiment of FIG. 1, the substrate 24 comprises a silicon on insulator (SOI) substrate including a silicon layer 24 a, an insulator layer 24 b and a silicon layer 24 c which are stacked on top of each other in a manner well known to those skilled in the art. The silicon layer 24 c may, for example, be of the fully depleted type.

It will be noted that the gate contact 18 is larger than the length of the transistor 10 (said length defined by the size of the gate region 12 and a channel region 27 provided in the silicon layer 24 c). The source region 14 and drain region 16 are also provided in the silicon layer 24 c on either side of the channel region 27, but at least part of each of the source region 14 and drain region 16 is located vertically underneath the gate contact 18 due to the relatively small pitch scaling of the transistors 10. This makes it difficult to place the source-drain contacts in the manner taught by the prior art to reach the source region 14 and drain region 16 from above the transistor in a manner similar to that provided by the gate contact 18. The transistors 10 instead utilize source-drain contacts 20 which extend from below the transistor to make electrical contact to the source region 14 and/or drain region 16. The junction 22 is formed at the same level as the silicon layer 24 c in order to provide an electrical interconnection of the source-drain contact 20 to the source region 14 and/or drain region 16. The junction 22 includes a silicide contact 22 a at the top of the gate contact 18 in order to ensure a good electrical connection of low resistivity. The gate contact 18 extends through insulating materials 26 and 28 to reach the gate region 12. These insulating materials form the pre-metal dielectric region as known to those skilled in the art. The top surface of the insulating materials, along with the top surface of the gate contact 18, is planarized to provide a co-planar surface 30 configured to support further back end of line (BEOL) fabrication (such as the addition of metallization layers and pads) known to those skilled in the art.

Reference is now made to FIGS. 2A-2M which illustrate process steps for forming the integrated circuit shown in FIG. 1.

FIG. 2A shows the starting silicon on insulator (SOI) substrate 24 (provided in a wafer format) including a base substrate layer 24 a (for example, formed of silicon), an insulator layer 24 b and a semiconductor layer 24 c (for example, made of silicon or silicon germanium). The semiconductor layer 24 c is preferably patterned in the shape of a fin structure as known to those skilled in the art and familiar with the fabrication of FINFET-type transistors (see, FIG. 3B). The layers of substrate 24 are stacked with the insulator layer 24 b positioned in contact with and between the base substrate layer 24 a and the semiconductor layer 24 c. The semiconductor layer 24 c may be doped if desired or alternatively may be fully depleted.

FIG. 2B illustrates the deposition of an insulating material 26 over the wafer on a top surface of the substrate 24. If the semiconductor layer 24 c has been patterned to form fins, the insulating material 26 will fill areas between adjacent fins (not explicitly shown) and reach down to the insulator layer 24 b. In a preferred implementation, the insulating material 26 comprises a low-k dielectric material, for example, made of silicon nitride (SiN). The deposit of insulating material 26 is conformal, and thus a top surface of the material as deposited may not be planar. In such a case, a chemical-mechanical polishing (CMP) operation is performed to provide a planar top surface 34. A patterned mask 38 is then formed on the planar top surface 34, the mask 38 including a number of openings 40 which correspond to the locations where source-drain contacts 20 (see, FIG. 1) are to be formed. The material used for the patterned mask 38 may, for example, comprise a-Si material patterned using a lithographic etch in a manner well known to those skilled in the art.

A highly direction etch as known in the art is then used with the mask 38 to etch openings 42 a which extend completely through the insulating material 26, completely through the semiconductor layer 24 c and into (but not completely through) the insulator layer 24 b of the substrate 24. The result of the etch process is shown in FIG. 2C.

The openings 42 a are then filled with a metal conductor material. As an example, the metal conductor material may comprise Tungsten (W). The fill operation may be performed using a chemical vapor deposition (CVD), as known in the art. The result of this deposition will produce metal material covering the mask 38. A chemical-mechanical polishing (CMP) operation is performed to remove the metal down to the level of the mask 38. An etch which is selective to remove the metal material, such as a plasma etch using BC13 chemistry, as known in the art, is then performed to recess the deposited metal material in the openings 42 to produce the source-drain contacts 20. The recess process removes the metal material down to a level at or below the interface between the insulator layer 24 b and the semiconductor layer 24 c and leaves openings 42 b. The result of the fill, polish and etch process is shown in FIG. 2D.

It will be noted that the openings 42 b extend through the semiconductor layer 24 c and thus sides of the openings 42 b at the semiconductor layer 24 c are exposed. A doping implantation is then performed through the openings 42 b at the exposed sides to dope the semiconductor layer 24 c and form the source regions 14 and drain regions 16 (with the portion of the semiconductor layer 24 c between the doped source region 14 and drain region 16 defining the channel region 27. The result of the dopant implantation is shown in FIG. 2E.

The junction 22, including if desired a silicide contact 22 a, is then formed within each opening 42 b at the top of each gate contact 18. A number of different techniques are available to form the junction 22. In one embodiment, a liner silicide contact with an epitaxial growth may be formed within each opening 42 b (see, FIG. 2F1 with epitaxial semiconductor region 22 b and silicide region 22 a). In another embodiment, a metal insulator semiconductor (MIS) type junction contact may be formed within each opening 42 b (see, FIG. 2F2 with metal region 22 c and thin insulator 22 d). The techniques for forming the junction 22 within each opening 42 b are well known in the art. The junction 22 functions to electrically connect the source-drain contact 20 within each opening 42 b to the source region 14 and drain region 16 on either side of the opening 42 b. The result of junction 22 formation is shown in FIG. 2F to leave openings 42 c.

In the following process steps, the junction 22 of the liner silicide contact type (FIG. 2F1) is shown, but it will be understood that the process steps are equally applicable to use of the junction of the MIS type (FIG. 2F2).

The openings 42 c are then filled by a deposit of an insulating oxide material 28. The oxide material may comprise, for example, a flowable oxide and the deposit may be made using a chemical vapor deposition (CVD) followed by a steam anneal in a manner well known to those skilled in the art. The deposit of the material 28 will produce material covering the mask 38. A chemical-mechanical polishing (CMP) operation is performed to remove the material down to the level of the mask 38. The result of the deposit and polish operations is shown in FIG. 2G with planar top surface 48.

A patterned mask 46 is then formed on the planar top surface 48 of the wafer, the mask 46 including an opening 50 which corresponds to the locations where the transistors 10 are to be formed (see, FIG. 1). The material used for the patterned mask 46 may, for example, comprise organic material patterned using a lithographic etch in a manner well known to those skilled in the art. An etch is then used with the mask 46 to etch openings 52 a which remove the mask 38 in the area of the transistors 10. The result of the patterned mask 46 formation and subsequent etch is shown in FIG. 2H.

The mask 46 is then removed. A conformal deposit of a spacer material is then made on the wafer and within the openings 52 a. A highly directional etch, such as an RIE, is then performed to remove the planar portions of that conformal deposit. The result of that etch is to produce sidewall spacers 60 on the side walls of the openings 52 a. The region between adjacent spacers 60 defines an opening 52 b. The result of the conformal deposit and subsequent etch is shown in FIG. 21.

The spacers 60 with openings 52 b are the used as a mask and a highly directional etch is performed to etch openings 52 c which extend completely through the insulating material 26 to reach the a top surface of semiconductor layer 24 c at the location of the channel region 27. This corresponds to the location of the gate regions 12 for the transistors 10 (see, FIG. 1). The etch may comprise, for example, a fluorine based plasma etch as known in the art. The result of the etch process is shown in FIG. 2J.

A thin layer of a high-k dielectric material is then deposited to line the openings 52 c. This layer defines the gate dielectric 68 for the transistor. The high-k dielectric material may comprise HfO2 deposited using an atomic layer deposition (ALD) process as known in the art. The openings 52 c are then filled with a metal conductor material which comprises a work function metal (WFM) as known in the art. As an example, the metal conductor material may comprise TiN, TiC, TiAl, TaN, TaC, and the like. The fill operation may be performed using a chemical vapor deposition (CVD) as known in the art. The result of this deposition will produce metal material covering the wafer. A chemical-mechanical polishing (CMP) operation is performed to remove the metal down to the level of the mask 38. An etch which is selective to the metal material, as known in the art, is then performed to recess the deposited metal material in the openings 52 c to produce the gate region 12 (see, for example, U.S. Pat. Nos. 8,617,973 or 8,679,909, the disclosures of which are incorporated by reference). The recess process removes a large portion of the metal material within the openings 52 c and leaves openings 52 d. The result of the line, fill, polish and etch process is shown in FIG. 2K.

A selective etch as known the art is then performed to remove both the spacers 60 and the etched portion of the insulating material 26 which defines the openings 52 d down to the level of the gate region 12. The result of the etch process is shown in FIG. 2L to produce an opening 52 e at each transistor gate region 12.

An optional oxidation of the wafer is then performed. This operation expands the size of each opening 52 e.

The openings 52 e are then filled with a metal conductor material 54. As an example, the metal conductor material may comprise Tungsten (W). The fill operation may be performed using a chemical vapor deposition (CVD), as known in the art. The result of this deposition will produce metal material covering the wafer. See, FIG. 2M. A chemical-mechanical polishing (CMP) operation is performed to remove the metal down to a desired thickness of the integrated circuit. The result of the polish process produces the integrated circuit with gate contacts 18 as shown in FIG. 1.

It will be understood that the integrated circuit of FIG. 1 may be subjected to further processing including the formation of contacts extending from the top surface of the wafer downward through the premetal dielectric (layers 26 and or 28) to make electrical contact to the source-drain contacts 20. Reference is now also made to FIG. 3A. Those skilled in the art understand that such contacts would be made by an electrical via 60 structure positioned away from the gate contact 18 and extending downward through the insulating layer(s) forming the pre-metal dielectric to reach the source-drain contact 20. Additionally, an electrical via 62 structure may be provided to make electrical contact to the gate contacts 16. FIG. 3A shows a plan view of the integrated circuit showing an exemplary location of the vias 60 and 62 relative to the transistors 10 in general and the gate contact 18 and source-drain contact 20 in particular.

The implementation of FIG. 1, and the process of FIGS. 2A-2M, utilizes a substrate 24 of the SOI type. The advantages of the buried source-drain contact 20 for transistor layouts with small pitch scaling can be achieved with other substrate types as well.

Reference is now made to FIG. 4 which illustrates a cross-sectional diagram of a plurality of transistors utilizing a buried contact made to the transistor source-drain regions where the substrate is of a bulk semiconductor type. Like reference numbers refer to like or similar parts. The transistors 10 are formed on and in a substrate 124. In the illustrated embodiment of FIG. 4, the substrate 124 comprises a bulk type substrate including a silicon layer 124 a, a silicon-germanium layer 124 b and a silicon layer 124 c which are stacked on top of each other in a manner well known to those skilled in the art. Because the silicon-germanium layer 124 b is semiconductive, the buried source-drain contacts 20 are insulated from the silicon-germanium layer 124 b within an insulated trench 126.

Reference is now made to FIGS. 5A-5G, plus FIGS. 2E-2M, which illustrate process steps for forming the integrated circuit shown in FIG. 4.

FIG. 5A shows the starting bulk substrate 124 (provided in a wafer format) including a base substrate layer 124 a (for example, formed of silicon), a first semiconductor layer 124 b (for example, formed of silicon-germanium) and a second semiconductor layer 124 c (for example, made of silicon). The semiconductor layer 124 c is preferably patterned in the shape of a fin structure as known to those skilled in the art and familiar with the fabrication of FINFET-type transistors (see, for example, the fins illustrated in FIG. 3B). The layers of substrate 124 are stacked with the first semiconductor layer 124 b positioned in contact with and between the base substrate layer 124 a and the second semiconductor layer 124 c. The second semiconductor layer 124 c may be doped if desired or alternatively may be fully depleted.

FIG. 5B illustrates the deposition of an insulating material 26 over the wafer on a top surface of the substrate 124. If the second semiconductor layer 124 c has been patterned to form fins, the insulating material 26 will fill areas between adjacent fins (not explicitly shown) and reach down to the second semiconductor layer 124 b. In a preferred implementation, the insulating material 26 comprises a low-k dielectric material, for example, made of silicon nitride (SiN). The deposit of insulating material 26 is conformal, and thus a top surface of the deposited material may not be planar. In such a case, a chemical-mechanical polishing (CMP) operation is performed to provide a planar top surface 34. A patterned mask 38 is then formed on the planar top surface 34, the mask 38 including a number of openings 40 which correspond to the locations where source-drain contacts 20 (see, FIG. 4) are to be formed. The material used for the patterned mask 38 may, for example, comprise a-Si material patterned using a lithographic etch in a manner well known to those skilled in the art.

A highly direction etch as known to those skilled in the art is then used with the mask 38 to etch openings 142 a which extend completely through the insulating material 26, completely through the semiconductor layer 124 c and into (but not completely through) the semiconductor layer 124 b of the substrate 124. The result of the etch process is shown in FIG. 5C.

An isotropic etch of the first semiconductor layer 124 b of the substrate 124 is then performed. This etch is selective as to silicon so as to suppress etching of the semiconductor layer 124 c. The isotropic etch may comprise a gap phase HCl dry etch as known to those skilled in the art. The result of the etch process is shown in FIG. 5D to form trench regions 142 b in the first semiconductor layer 124 b of the substrate 124 aligned with the etch openings 142 a.

The trench regions 142 b are then filled with an insulating material 126. The material 126 may, for example, comprise an oxide material such as silicon oxide. The filling of the trench regions 142 b with the oxide material 126 may be accomplished using a thermal oxide growth as known to those skilled in the art. The result of the fill operation is shown in FIG. 5E. It will be understood, although not explicitly shown in FIG. 5E, that the material 126 filling the trench regions 142 b may preferably overfill into the openings 142 a.

A highly directional etch (as known in the art) which preferentially removes the oxide insulating material 126 is then performed form trench openings 142 c in the oxide insulating material 126. The result of the etch is shown in FIG. 5F.

The openings 142 c are then filled with a metal conductor material. As an example, the metal conductor material may comprise Tungsten (W). The fill operation may be performed using a chemical vapor deposition (CVD), as known in the art. The result of this deposition will produce metal material covering the mask 38. A chemical-mechanical polishing (CMP) operation is performed to remove the metal down to the level of the mask 38. An etch which is selective to remove the metal material, such as a BC13 plasma etch, as known in the art, is then performed to recess the deposited metal material in the openings 142 c to produce the source-drain contacts 20. The recess process removes the metal material down to a level at or below the interface between the semiconductor layer 124 b and the semiconductor layer 124 c and leaves openings 42 b. The result of the fill, polish and etch process is shown in FIG. 5G.

At this point, the fabrication process continues with FIGS. 2E to 2M so as to produce the integrated circuit shown in FIG. 4. The fabrication process steps of FIG. 2E-2M are incorporated by reference to follow the fabrication process steps of FIGS. 5A-5G.

Reference is now made to FIG. 6 which illustrates a cross-sectional diagram of a plurality of transistors utilizing a buried contact made to the transistor source-drain regions where the substrate is of a bulk semiconductor type. Like reference numbers refer to like or similar parts. The transistors 10 are formed on and in a substrate 224. In the illustrated embodiment of FIG. 6, the substrate 224 comprises a bulk type substrate for example formed of silicon in a manner well known to those skilled in the art. Because the silicon substrate 224 is semiconductive, the buried source-drain contacts 20 are insulated from the silicon substrate 224 within an insulated trench 226.

Reference is now made to FIGS. 7A-7I, plus FIGS. 2E-2M, which illustrate process steps for forming the integrated circuit shown in FIG. 6.

FIG. 7A shows the starting bulk substrate 224 (provided in a wafer format) that is formed, for example, of silicon. The semiconductor substrate 224 may be doped as desired or required for the given application. The top surface of the substrate 224 may be patterned, if desired, to for fins (see, for example, FIG. 3B).

FIG. 7B illustrates the deposition of an insulating material 26 over the wafer on a top surface of the substrate 224. In a preferred implementation, the insulating material 26 comprises a low-k dielectric material, for example, made of silicon nitride (SiN). The deposit of insulating material 26 is conformal, and thus a top surface of the deposited material may not be planar. In such a case, a chemical-mechanical polishing (CMP) operation is performed to provide a planar top surface 34. A patterned mask 38 is then formed on the planar top surface 34, the mask 38 including a number of openings 40 which correspond to the locations where source-drain contacts 20 (see, FIG. 6) are to be formed. The material used for the patterned mask 38 may, for example, comprise a-Si material patterned using a lithographic etch in a manner well known to those skilled in the art.

A highly direction etch as known to those skilled in the art is then used with the mask 38 to etch openings 242 a which extend completely through the insulating material 26 and into (but not completely through) the semiconductor substrate 224. The result of the etch process is shown in FIG. 7C.

A conformal deposit of a spacer material is then made on the wafer and within the openings 242 a. A highly directional etch, such as an RIE, is then performed to remove the planar portions of that conformal deposit. The result of that etch is to produce sidewall spacers 260 on the side walls of the openings 242 a. The region between adjacent spacers 260 defines an opening 242 b. The result of the conformal deposit and subsequent etch is shown in FIG. 7D.

The spacers 260 with openings 242 b are the used as a mask and a highly directional etch as known in the art is performed to etch openings 242 c which extend deeper into the semiconductor substrate 224. These openings 242 c correspond to the locations of the source-drain contacts 20 12 for the transistors 10 (see, FIG. 6). The result of the etch process is shown in FIG. 7E.

An isotropic etch of the semiconductor substrate 224 is then performed through the openings 242 c. The isotropic etch may comprise an SF6/C12 based plasma etch as known to those skilled in the art. The result of the etch process is shown in FIG. 7F to form trench regions 242 d in the substrate 224 aligned with the etch openings 242 c.

The spacers 260 are then removed to form openings 242 e aligned with the trench regions 242 d. The trench regions 242 d are then filled with an insulating material 226. The material 226 may, for example, comprise an oxide material such as silicon oxide. The filling of the trench regions 242 d with the oxide material 226 may be accomplished using a thermal oxide growth as known to those skilled in the art. The result of the fill operation is shown in FIG. 7G. It will be understood, although not explicitly shown in FIG. 7G, that the material 226 filling the trench regions 242 d may preferably overfill into the openings 242 e.

A highly directional etch (as known in the art) which preferentially removes the oxide insulating material 226 is then performed to form trench openings 242 f in the oxide insulating material 226. The result of the etch is shown in FIG. 7H.

The openings 242 f are then filled with a metal conductor material. As an example, the metal conductor material may comprise Tungsten (W). The fill operation may be performed using a chemical vapor deposition (CVD), as known in the art. The result of this deposition will produce metal material covering the mask 38. A chemical-mechanical polishing (CMP) operation is performed to remove the metal down to the level of the mask 38. An etch which is selective to remove the metal material, such as a BC13 plasma etch, as known in the art, is then performed to recess the deposited metal material in the openings 242 f to produce the source-drain contacts 20. The recess process removes the metal material down to a level at or below the location of the filled trenches and leaves openings 42 b. The result of the fill, polish and etch process is shown in FIG. 7I.

At this point, the fabrication process continues with FIGS. 2E to 2M so as to produce the integrated circuit shown in FIG. 6. The fabrication process steps of FIG. 2E-2M are incorporated by reference to follow the fabrication process steps of FIGS. 7A-7I.

The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of one or more exemplary embodiments of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims. 

What is claimed is:
 1. A method, comprising: forming a trench in a substrate; at least partially filing said trench with a metal material form a source contact buried in the substrate but exposing a side wall of the substrate; doping the exposed side wall of the substrate to define a source region adjacent a channel region; providing a gate dielectric on top of the channel region; and forming a gate electrode on the gate dielectric.
 2. The method of claim 1, wherein the substrate comprises a silicon on insulator (SOI) substrate including insulator layer between a base substrate layer and a semiconductor layer, wherein the source region and channel region are located in the semiconductor layer, and wherein forming the trench comprises forming the trench to extend through the semiconductor layer and into the insulator layer.
 3. The method of claim 2, further comprising defining a plurality of fins in the semiconductor layer, each fin including said source region and channel region.
 4. The method of claim 2, further comprising forming a conductive junction in the trench to electrically connect a top of the buried source contact to a side of the source region, said conductive junction provided at a same level as the semiconductor layer.
 5. The method of claim 4, further comprising forming a silicide region between the top of the buried source contact and a bottom of the conductive junction.
 6. The method of claim 1, wherein the substrate comprises a bulk substrate including first semiconductor layer between a base substrate layer and a second semiconductor layer, wherein the source region and channel region are located in the second semiconductor layer, and wherein forming the trench comprises forming the trench to extend through the second semiconductor layer and into the first semiconductor layer.
 7. The method of claim 6, further comprising defining a plurality of fins in the second semiconductor layer, each fin including said source region and channel region.
 8. The method of claim 6, further comprising forming an insulating layer in the trench to separate the metal material of the source contact from at least the first semiconductor layer.
 9. The method of claim 6, further comprising forming a conductive junction in the trench to electrically connect a top of the buried source contact to a side of the source region, said conductive junction provided at a same level as the second semiconductor layer.
 10. The method of claim 9, further comprising forming a silicide region between the top of the buried source contact and a bottom of the conductive junction.
 11. The method of claim 1 wherein the substrate comprises a bulk substrate of semiconductor material, wherein the source region and channel region are located in a top portion of the bulk substrate, and wherein forming the trench comprises forming the trench to extend into the bulk substrate to a depth below the top portion.
 12. The method of claim 11, further comprising defining a plurality of fins in the top portion of the bulk substrate, each fin including said source region and channel region.
 13. The method of claim 11, further comprising forming an insulating layer in the trench to separate the metal material of the source contact from the bulk substrate.
 14. The method of claim 11, further comprising forming a conductive junction in the trench to electrically connect a top of the buried source contact to a side of the source region, said conductive junction provided at a same level as the top portion.
 15. The method of claim 14, further comprising forming a silicide region between the top of the buried source contact and a bottom of the conductive junction.
 16. The method of claim 1, further comprising forming a gate contact extending from above the gate electrode to make electrical contact with the gate electrode, said gate contact laterally extending over the source region.
 17. The method of claim 16, further comprising: forming a premetal dielectric region over the substrate within which the gate contact is located; and forming a via extending through said premetal dielectric region to make electrical contact with said buried source contact. 